The present invention relates to an RF coil used in a transmitting or receiving system for a nuclear magnetic resonance (hereinbelow abbreviated to NMR) imaging device for obtaining a tomographic image of a desired portion of a body to be examined (human body) utilizing NMR phenomena, in which sensitive directions of two conductive loops intersect perpendicularly to each other, forming a pair, and inparticular to an RF coil for an NMR imaging device capable of reducing coupling between the two conductive loops described above.
An NMR imaging device is composed of magnetic field generating means giving a static magnetic field and a gradient magnetic field in a direction perpendicular to the body axis of a body to be examined; a transmitting system for irradiating the body, to be examined with an RF signal in order to produce NMR in nuclei of atoms constituting living body structure thereof; a receiving system for detecting an RF signal emitted by the NMR described above; and a signal processing system performing an image reproducing operation, using the RF signal detected by this receiving system. It was so constructed that a RF signal having a frequency capable of exciting NMR was applied to a body to be examined by means of the RF coil in the transmitting system, while giving a uniform static magnetic filed thereto by using the static magnetic field generating means and an NMR signal emitted by the body to be examined in this way was detected by the RF coil in the receiving system. At this time, in order to specify the position, where the NMR signal was emitted by the body to be examined described above, imaging was effected by giving further a gradient magnetic field by using the gradient magnetic field generating means.
As an RF coil in such an NMR imaging device, heretofore there was known a coil, in which one conductive loop, e.g. a solenoid coil or a saddle coil was used, which received the NMR signal in one direction. On the contrary, there was known another coil, in which two conductive loops intersected so that sensitive directions there of were perpendicular to each other, forming a pair which received the NMR signal in two directions. The sensitive direction is in accordance with the direction, in which the magnetic field is generated. The latter RF coil consisting of combined two conductive loops is called, Quardrature Detection Coils, hereinbelow abbreviated to QD coils. As a prior art QD coil, a combination of a saddle coil with another saddle coil has been proposed e.g. for an NMR imaging device by the horizontal magnetic field method, in which the static magnetic field is generated parallely to the body axis of a human body. However, when the combination of a saddle coil with another saddle coil was used, in particlar in an NMR imaging device by the vertical magnetic field method, in which the staic magnetic field is generated in the up- and downward direction, the direction of the static magnetic field was in accordance with the reception direction and it was impossible to receive the signal with a high sensitivity. Therefore, recently a combination of a solenoic coil with a saddle coil has been proposed e.g. for the QD coil by the vertical magnetic field method.
However, in such a prio art RF coil, in particular in a QD coil by the vertical magnetic field method, since it was a combination of two different coils such as a solenoid coil and a saddle coil, it happened that coupling was produced between the different coils. Here the coupling means that when an RF current flows through one of the coils, the RF current leaks to the other coil and magnetic filed is produced further by the leak current. When such a coupling is produced, each of the coils is a lood of the other and acts as loss for each coil. This lowered the sensitivity of the RF coil as a whole. Therefore it was caused that the S/N ratio of the obtained image was lowered.
Here, as a cause of the coupling between the RF coils described above, capacitive coupling, by which parasitic capacitance is produced between the two coils due to the fact that the interval there between at the intersection is as small as several mm and current flowing through one of them leaks to the other, and inductive coupling, by which unbalance is produced with respect to the magnetic flux of one of the coils by the magnetic flux generated by the other, are conceivable. The inductive coupling can be reduced by adjusting the unbalance in the magnetic flux by disposing a plate made of a conductive material, e.g. copper, in the proximity of the coils and therefore it gives rise to no serious problem. On the other hand, the capacitive coupling can be reduced by decreasing parasitic capacitance formed between the coils by increasing the interval therebetween at the intersection. That is, when two plane conductive plates A.sub.1 and A.sub.2 are located closely and parallelly to each other (corresponding to the intersecting portion of the two coil conductors), as indicated in FIG. 5, denoting the interval between the two plates A.sub.1 and A.sub.2 by d, the area of the plane conductive plates A.sub.1 and A.sub.2 by S, and the dielectric constant of the space therebetween by .epsilon., the electric capacitance between the two plane conductive plates A.sub.1 and A.sub.2 described above is given by; ##EQU1## As clearly seen from this Eq (1), the electric capacitance C between the two plane conductive plates A.sub.1 and A.sub.2 is decreased by increasing the interval d therebetween.
Consequently, heretofore, the capacitive coupling was reduced by decreasing parasitic capacitance formed by the two coils by increasing the interval therebetween at the intersection. However, in this case, the interval between the two coils should be increased with increasing NMR frequency and thus the whole sige of the RF coils was increased. Further the distance of at least one of the coils was great from the body to be examined which lowered further the sensitivity and worsened the S/N ratio.